Student Jobs - Hours Per Week?

Of course i would be interested to work only 12-15 hours/week, so i can have more time to learn for school or free time, but after i spent a year working full time as an engineer, i don't know how it would look like on my CV a part-time job for 12-15 hours / week
I know that i cannot do it both (work and school) with the same devotion, only if i will find something related to my field of study and my already gained work experience.

I'm sure someone would put two and two together because you would have your part time job at the same time as you were at university. You could always explain at interviews when asked and surely it would look good that despite studying you kept up some kind of job to support yourself?

Just wondering how many hours of waged employment university students find they are able to comfortably work per week around studying and socialising